1. Field of the Invention
The present invention relates to improvements of a multiple-step transmission which has at least seven forward speed steps or gear positions.
2. Discussion of Related Art
For vehicles, there are widely used automatic transmissions which have a plurality of planetary gear sets, a plurality of clutches, and a plurality of brakes. As those automatic transmissions, there have been proposed multiple-step transmissions which have at least six forward speed steps or gear positions. Examples of those multiple-step transmissions are disclosed by Patent Document 1 (JP-A-2002-206601), Patent Document 2 (JP-A-8-105496), Patent Document 3 (JP-A-2000-199549), Patent Document 4 (JP-A-2000-266138), Patent Document 5 (JP-A-2001-82555), Patent Document 6 (JP-A-2002-227940), Patent Document 7 (JP-A-2002-295609), and Patent Document 8 (JP-B2-2956173). In those examples, three or four planetary gear sets are employed to provide from six to twelve forward speed steps.
Meanwhile, the multiple-step transmissions are required to (a) have a sufficiently wide range of speed ratio in total, (b) have respective speed ratios whose steps have substantially same ratios or nearly same ratios, (c) have the least number of clutches and brakes, (d) be able to change easily respective operating states of the clutches and brakes, for the purpose of changing speed steps, (e) have the least number of planetary gear sets, and (f) employ respective planetary gear sets whose gear ratios ρ {i.e., (number of teeth of sun gear)/(number of teeth of ring gear)} fall in the range of from about 0.3 to about 0.6, for the purpose of enjoying a reduced size. However, it is difficult to meet all of the above-indicated requirements and accordingly the conventional transmissions have not been sufficiently satisfactory. For example, the multiple-step transmission disclosed by the above-indicated Patent Document 1 employs seven engaging elements, i.e., clutches and brakes, and needs to change respective operating states of at most four engaging elements, for the purpose of changing speed steps. Therefore, this transmission needs a complex and highly accurate speed-change controllability, and may produce a speed-change shock. In addition, the gear ratios ρ of the planetary gear sets of the transmission falls in the range of from 0.273 to 0.778, and accordingly the transmission can hardly be constructed in a small size.